Polarization-maintaining (PM) optical fibers are configured to preserve the polarization direction of the guided light as it travels down the fiber. PM optical fibers are used in certain types of applications that use polarized light, such as optical-fiber-based interferometers, optical fiber sensors, optical fiber gyroscopes, high-power optical fiber lasers, some types of quantum key distribution systems, and for the light-modulating sections of optical telecommunication systems.
There are a number of different geometries for PM optical fibers that create the fiber's PM capability. An example PM optical fiber comprises a central GeO2-doped core along with two stress-applying members in the cladding that create birefringence, resulting in an optical fiber with excellent PM properties. Examples of such a PM optical fiber are disclosed in U.S. Pat. No. 7,697,809, which is incorporated by reference herein. Other example PM optical fibers are disclosed in U.S. Pat. No. 7,412,142, which is incorporated by reference herein.
However, present-day PM optical fibers have two main shortcomings. The first is that doped-core PM optical fibers are relatively sensitive to aging effects as compared to a pure silica core. The second is that they operate over a relatively limited spectral band.